1. Field of the Invention
The present invention relates to an image fixing device used in a machine employing an electrophotographic method such as a copying machine, a facsimile apparatus or a printer, and in particular relates to an image fixing device having a heat roller and fixed pressure means.
2. Discussion of the Related Art
In a copying machine and the like that utilizes an electrophotographic method, it is necessary to fix an unfixed toner image transferred on a recording sheet to form a permanent image. Therefore, fixing methods such as vapor fixing, cold pressure fixing and heat fusing have been conventionally used for permanent image formation. However, when the vapor fixing method is employed, a solvent vaporizes and diffuses, which causes an unpleasant odor. The cold pressure fixing method is inferior to other methods in fixing capability, and there is an economical problem such that the cold pressure fixing requires use of expensive pressure sensitive toner, for example, capsule toner. Both cold pressure fixing and vapor fixing are not widely used at present.
Accordingly, the heat fusing method, which melts the toner by heating and fixes it on the recording sheet, is widely employed in fixing an unfixed toner image. There are various types of heat fusing devices which bring the heat fusing method into practice. In particular, devices employing a heat roller method are generally used. As shown in FIG. 3, a device of such type comprises a heat roller 1, which is rotatably mounted, and a pressure roller 2. The heat roller 1 is made of a metal cylindrical core 3, a heater 4, such as an infrared ray lamp disposed inside of the metal cylindrical core 3, and a releasing layer 5 covering the peripheral surface of the metal cylindrical core 3. The releasing layer 5 is formed to prevent attachment of toner, which has transferred from the surface of the recording sheet, to the peripheral surface of the core 3 and is made of heat-resisting material such as fluororesin, silicone rubber or silicone resin.
The pressure roller 2 is disposed to contact the heat roller 1 and apply pressure thereto, and consists of a core 6 and a releasing layer 7, made of heat-resisting and elastic material, coating the peripheral surface of the core 6 to improve the paper stripping capability. As the heat-resisting and elastic material, for example, silicone rubber or fluororubber can be used. By rotating the heat roller 1 and pressure roller 2, a recording sheet 9 on which an unfixed toner image 8 is formed is passed through a nip area between the heat roller 1 and pressure roller 2, where the heat and pressure are applied to the recording sheet 9 to fix the toner image 8.
In comparison with other heat fusing methods, such as radiant fusing and oven fusing, the heat efficiency of the heat roller method is higher. Consequently, a smaller amount of electric power is consumed and high-speed fixing can be performed. Even if the recording sheets are jammed, the temperature is easily controlled so that the temperature of the recording sheet is not much higher than that of the heat roller. Accordingly, since there is little possibility of causing a fire, the heat roller method is moreover advantageous and is most widely used at present.
In general, to fix a toner image sufficiently, it is necessary to provide a nip width (the length of a portion where the heat roller 1 and the pressure roller 2 are in contact with each other in the direction of movement of the recording sheet 9) of, at least, about 4 mm. However, in the conventional image fixing device employing the heat roller method, since both heat roller 1 and pressure roller 2 are cylindrical, the diameters of heat roller 1 and pressure roller 2 must be large. As a result, the device as a whole necessarily becomes bulky and complex. Further, a problem occurs because the time required for raising the temperature of the device until fixing becomes practicable (hereinafter, referred to as warmup time) is inevitably long.
To overcome the above problems, image fixing devices using a fixed heat-resisting and elastic pressure applying member (pressure applying means)instead of the rotatable pressure roller 2 have been proposed (Japanese Patent Application Unexamined Publication No. Sho. 59-68766 (1984), Japanese Utility Model Application Unexamined Publication No. Sho. 60-8966 (1985), Japanese Utility Model Application Unexamined Publication No. Sho. 60-33362 (1985), Japanese Patent Application Unexamined Publication No. Sho. 62-135865 (1987), Japanese Utility Model Application Unexamined Publication No. Sho. 63-62862 (1988), Japanese Patent Application Unexamined Publication No. Sho. 50-134655 (1975), Japanese Patent Application Unexamined Publication No. Sho. 50-57444 (1975), Japanese Patent Application Unexamined Publication No. Sho. 61-11773 (1976), Japanese Utility Model Publication No. Hei. 4-52770 (1992) and Japanese Utility Model Application Unexamined Publication No. Sho. 63-62861 (1988)). In these devices, a portion of the surface of the heat-resisting and elastic pressure applying member contacting heat roller 1 is circular. Unlike the cylindrical pressure roller 2, the circular surface of the heat-resisting and elastic pressure applying member always contacts the heat roller 1. Therefore, the radius of curvature of the circular surface can be set regardless of the size of the heat-resisting and elastic pressure applying member, thus enabling miniaturization of the device.
In these devices, however, because the heat-resisting and elastic pressure applying member is fastened, it is hard for the end of the recording sheet 9 to enter the nip area, though the heat roller 1 is rotating. Even if entered, there are cases where the carrying speed of the recording sheet 9 is unstable. Further, a difference in carrying speed occurs in the longitudinal direction of the heat roller 1 (the direction of width of the recording sheet 9), which sometimes results in displacement of the recording sheet 9 in either side of the direction.